This invention provides a system for the determination of sorption bed characteristics. More particularly, the invention provides a method and apparatus for determining thermal characteristics of a sorption bed through the measurement of temperatures of the vessel walls which contain the bed. The invention has application in improving the efficiency of adsorption vessels, solids drying apparatus, and like devices. Moreover, the invention has application as a diagnostic device for such apparatus.
In accord with teachings of the prior art, an apparatus for the adsorption treatment of fluids typically comprises a vessel having a fluid feed entrance and a fluid product exit. Disposed within the vessel, is a sorbent material designed for reducing sorbate concentration in a feed fluid passing through the vessel. According to a conventional mode of operation, in the initial sorption stage, a fluid stream containing a dilute species to be removed, i.e, the sorbate, is introduced to the vessel via the fluid feed entrance and passed along the flow path of the bed. Inside the bed, as sorbate is removed from the fluid, a sorption wave or "front" is created which passes along the flow path in the same direction as the fluid flow, but at a much slower rate.
According to recent characterizations of sorption bed mechanisms, a sorption front is defined as the bed region where occur changes in sorbent loading and sorbate content of the fluid phase. The front's upstream side is bounded by a bed region characterized by sorbent loadings, sorbate to fluid feed mole ratios, and temperatures characteristic of equilibrium between the sorbent material and the feed. On its downstream side, the sorption front is bounded by a bed region having properties characteristic of equilibrium between the sorbent material and the substantially sorbate-free fluid product. As the downstream boundary of the sorption front approaches the bed exit, the concentration of sorbate in the product begins to rise. Front boundaries are generally not well defined, but rather comprise regions in which the specific bed and fluid characteristics asymptotically approach equilibrium.
According to the conventional techniques for the control of the sorption stage, concentration of sorbate in the product at the exit of the vessel is monitored. When this concentration exceeds some predetermined amount, sorption is discontinued, and the bed is regenerated.
In the regeneration stage, a regenerant hot fluid is passed through the bed in a co-current, or more commonly, a countercurrent direction. The high temperature of the regenerant effects a desorption front in the bed, which front drives sorbate from the sorbent material and into the flowing regenerant stream. This process is continued until the bed is substantially sorbate-free, as typically revealed by monitoring waste at the veseel exit for the emergence of substantially sorbate-free regenerant fluid.
Sorption bed systems of the type described above are known as "thermal swing" systems because they are regenerated with heat. These systems have been widely utilized in various industries. For example, in the preparation of air for use in pneumatic systems, water vapor is often first removed from the air by its passage through activated alumina or zeolite. Natural gas is similarly treated before it is liquified or delivered to a pipeline. Activated carbon can be used to remove trace quantities of organic vapors from air in solvent recovery operations. Similarly, carbon dioxide, mercury, oxides of nitrogen and sulfur, and hydrogen sulfide may be removed from air or exhaust on zeolite molecular sieves or on activated carbon. Recently, thermally regenerable ion exchange resins have been utilized for removal of salts from water.
A drawback of the conventional adsorption bed operational techniques is the requirement that sorbate concentration at the vessel exit be monitored in order to adequately control passage of fluids through the vessel. Moreover, these techniques typically treat the adsorbent bed as a "black box." That is, the status of the sorption bed or its sensitivity to variations in gas composition, temperature and flow rate are generally wholly unknown to the operator. Consequently, to insure proper operation, a considerable excess of regeneration energy and frequent desiccant changeouts are standard procedure.
In one recent advance in sorption bed technology, Oliker, U.S. Pat. No. 4,324,564, discloses the modification of the cycle of operation of beds of thermal swing systems. In accord with the technique of that patent, it is possible to significantly reduce the quantity of head needed for regeneration, to increase the throughput of a bed of a given size, to decrease the size and thus the capital costs of a bed system required to achieve a given capacity, to upgrade the quality of the reduced sorbate-concentration product, and to provide greater security against break-through of feed during the sorption stage. The teachings of that patent are expressly incorporated by reference herein.
More particularly, the U.S. Pat. No. 4,324,564 discloses the "Four Front" method. As disclosed therein, regeneration of sorption beds occurs through the medium of moving fronts or regions in the sorption bed where changes in sorbent loading, temperature, and sorbate content of the gas occur. During regeneration, introduction of a hot gas stream into the bed creates a desorption front. This front, designated as an RW front, is bounded on its downstream side by bed conditions characteristic of equilibrium between the sorbent material and fluid waste ("W", sorbate-rich effluent"), and on its upstream side by bed conditions characteristic of equilibrium between the hot sorbent material and hot regenerant gs ("R"). Upon subsequent introduction of cooling fluid, another front is created, designated therein as a thermal front or a "PR" front, which moves more rapidly than the RW front. The thermal front can arise in several ways. When the hot regenerant gas contains a substantial concentration of sorbate (e.g., water) and the coolant is substantially sorbate-free (assumed for purposes of discussion to have characteristics similar to dry product gas "P"), a PR transition is created comprising a faster stripping front which effects removal of all or most of the sorbate on the hot bed in equilibrium with the hot regenerant fluid, and a thermal front which effects the major temperature transition. Upstream of this transition, the bed is in equilibrium with coolant (herein designated "P"), while downstream, the bed is in equilibrium with hot regenerant. If sorbate is present in the coolant, multiple fronts are produced which together form the PR transition, including one front which constitutes the major thermal front. When the regeneration and cooling are conducted using substantially sorbate-free gas, the PR transition is a pure thermal wave bounded on its downstream side by bed conditions characteristic of equilibrium between the sorbent and the hot regenerant gas, and on its upstream side by bed conditions characteristic of equilibrium between the sorbent and the cooling gas.
U.S. Pat. No. 4,324,564 discloses that a number of operational advantages including energy savings can be achieved by timing the introduction of the cooling gas prior to the breakthrough of the midpoint of the RW front. The preferred operation of the Four Front method for sorption bed regeneration introduces the cooling gas so that the thermal component of the PR transition will be in the last third of the bed or most preferably at the bed exit when the slower RW front is at or breaking through the bed exit.
In a relatd development, copending U.S. patent application Ser. No. 484,159, filed Apr. 12, 1983, commonly assigned herewith, discloses a method and apparatus for drying granular solids. In accord with the teachings of that application, a heated gas stream is directed through a volume of solids and followed by a cooling gas stream. Through alteration of the timing of the introduction of the cooling stream, with respect to that timing utilized in conventional drying techniques, the solids drying mechanism efficiently uses the sensible heat contained in the solids to supply heat of evaporation. The teachings of U.S. patent application Ser. No. 484,159 are incorporated herein.
The above-described work of Oliker provides inroads into obviating deficiencies of the prior art sorption technology. For example, in lieu of the standard practice of controlling fluid passage through the monitoring of sorbate concentration, Oliker utilizes a temperature sensing device placed within the sorbent bed. By enabling detection of the passage of fronts through the bed, these internal temperature sensors lead to improvements in bed efficiency and longevity.
An object of the present invention is to provide still further advances in the art of sorption bed operation. More particularly, an object of the invention is to provide a mechanism for monitoring sorption bed characteristics, e.g., thermal characteristics, without necessitating the placement of internal probes, e.g., either within the bed fluid exit or within the sorption bed itself. Another object is to provide a simple bed monitoring mechanism which can be retrofit on existing adsorption treatment apparatus without requiring substantial modification thereof. Still another object is to provide a sorption apparatus which does not require the frequent down-time typically caused by prematurely aged sorption beds. Further, an object of the invention is to obviate the necessity for the over-design of sorption apparatus. Other objects of the invention are evident in the discussion below.